1. Field of the Invention
The present invention relates generally to semiconductor fabrication. More particularly, the present invention relates to the creation of a diffusion barrier layer on a substrate using physical vapor deposition (PVD).
2. Background of the Invention
As microelectronics continue to miniaturize, interconnection performance, reliability, and power consumption have become increasingly important. Interest has grown in replacing aluminum alloys with lower resistivity and higher reliability metals. Copper offers a significant improvement over aluminum as a contact and interconnect material. For example, the resistivity of copper is about half of the resistivity of aluminum. However the use of copper as an interconnect material present various problems not encountered with the use of aluminum because of the inherent properties of copper and its reaction with silicon and other various dielectrics employed in semiconductor fabrication. One of the problems is that copper will likely result in diffusion in the silicon or dielectric layer when placed in contact with silicon or dielectric. This diffusion can lead to the destruction of the underlying circuitry as well as an increase in electromigration. In addition copper does not bond well with the silicon or most dielectrics.
To solve the problem associated with the use of copper, manufactures of semiconductors have employed the use of a metal barrier layer between the copper contacts and interconnects and underlying material layers. Typically the metal barrier layer has two layers composed of a barrier layer to prevent the diffusion of copper and electromigration, and a wetting layer to enhance the adhesion of copper to the substrate. While a single layer of metal could serve to prevent diffusion and increase wettability, commonly a metal nitride is used for the barrier layer. This is because an increase in the nitrogen content of the metal nitride increases the protection against diffusion of copper into the substrate and electromigration. However, nitride layers do not provide the wettability of a pure metal. As such, dual layers are used to form the metal barrier layer. For instance the metal barrier layer could be composed of layers of titanium nitrate (TiN) and titanium (Ti). Typically metal barrier layers are composed of TiN/Ti, TaN/Ta, WN/W, and MoN/Mo. In addition, the use of a metal nitrate layer provides good adhesion to the silicon or dielectric medium.
One method of manufacturing the metal barrier layer is through physical vapor deposition (PVD), or more specifically through a form of PVD called sputtering. Deposition through sputtering is accomplished in an enclosed chamber, with a target electrode composed of at least part of the material to be sputter deposited and a substrate. A noble non-reactive gas such as argon is streamed through the chamber and is ignited to provide a plasma source. Sputtered particles traverse the chamber and stick to the substrate, forming a metal layer. To deposit the compound material, the chamber is additionally filled with a reactive gas to provide the additional compound elements. Sputtered particles chemically react with the reactive gas and are together deposited on the substrate as the compound material. For instance, to deposit Tantalum Nitride (TaN), the target would be formed from Ta and the chamber would be filled with a mixture of nitrogen gas and argon. The metal barrier layer is typically formed by repeating the sputtering process twice. The first time, nitrogen gas is introduced into the chamber with a metal target to produce a metal nitrite layer. The second time, the nitrogen is removed and a non-reactive gas such as argon is introduced, to produce a pure metal layer.
The metal barrier layer, while necessary when using copper, presents problems since it is more resistive than copper and hence leads to slower signal propagation and a slower device. This is particularly true of the nitrite layer as an increase of nitrogen cause the metal nitrate to be more resistive. As such, the thickness of the nitrite layer needs to be kept to a minimum to reduce the resistance. Also as chips become more miniature and smaller process are used in fabricating the chips, thinner metal barrier layers are need. For instance, in a 65 nm process, it is desirable for the nitrate layer to have a thickness of 1 nm or less.
However, when using sputtering, it is often difficult to consistently produce a layer of either metal or metal nitrate that is less than 2 nm in thickness. Nitrogen flows during and after ignition limit the minimum thickness, resulting in different nitrogen incorporation and properties. Thus, the typical 2-step sputtering is not a reliable process in manufacturing metal nitrates layers that are loss than 1 nm thick as needed for 65 nm and smaller process used to fabricate semiconductors. There exist other methods for manufacturing a thin film of a metal nitrite that is less than 1 nm thick. For instance, a thin film metal nitrate can be deposited using Atomic Layer Deposition (ALD). However this process is much more expensive than sputtering. In addition, the use of ALD to produce a barrier layer has not been proven in terms of reliability. What is needed is a reliable, inexpensive and effective method to produce a barrier layer consisting of a metal nitride whose thickness is less than 1 nm.